Nasa Logo


+ Home
Solar Physics
       + Solar Cycle Prediction         + Magnetograph       + The Sun in Time       + The Hinode Mission      + The STEREO Mission

Planetarium Program
part I for
The Sun in Time

sun_in_time_ssm.jpg (10439 bytes)

These pages continue to evolve, with more images and text!!

Using the pointer, point out on the horizon the angular variation of the Sun's rising and setting position.

Welcome to the Planetarium.  Look up into our planetarium sky and see if you can find the one star which is visible in the daytime.  Does anyone know the name of our star, the Sun?  Sol is the name, and is the root for words such as solar system and solar energy. 

Did anyone see Sol rise this morning?  In what direction did it rise?   You might be surprised to learn that while the sun always rises in the easterly direction and sets in the westerly direction, it only rises directly east and sets directly west, twice a year.

{This presentation was given in the fall after the autumnal equinox, and must be adjusted for season.  After the autumnal equinox, when the Sun appears to cross the celestial equator, sunrise will be seen south of due east. After the vernal equinox, the sunrise position will be north of due east.}

Demonstrate with pointer, a position south of due east

This morning, the Sun rose about here.  Each day after the autumnal equinox, the Sun rises a bit farther to the south until it reaches its southernmost position at the time of Winter Solstice -- there's the word Sol again!   (For the southern hemisphere, the southernmost position of the Sun would mark the the summer solstice.)

The winter solstice occurs on or about December 21 and the northern hemisphere has its shortest day of the year.  After that date, the rising sun retraces its path northward along the eastern horizon until it eventually reaches its northernmost position.  On that day, around June 22nd, people here in the northern hemisphere of the Earth have their longest day of the year, a day called the Summer Solstice. 

Turn on the celestial meridian and point to it.

This line, which passses through the celestial poles and the zenith, is called the celestial meridian.  The meridian is an imaginary line dividing the sky into an Eastern half and a Western half.  When you got up this morning and looked at your clock, you might have thought, "oh, no!  It's seven a. m., I'm late!"  What does the a.m. stand for?   A.M. is an abbreviation for ante meridiem...not your Uncle Meridiem, but ante meridiem (wait for laughter to subside...the joke is intended as a mnemonic device to help students remember the word, ante).  Ante meridiem is from the Latin and means "before the meridian".  As the Sun crosses the meridian and moves into the western half of the sky, we switch to p.m., or post meridiem meaning "after the meridian". 

Point out altitude of the Sun on the celestial meridian at noon.

At noon today, the Sun will reach this position on the celestial meridian.  Did you know that here in Huntsville the Sun never reaches the zenith or, a position directly overhead?  In Huntsville, the Sun will reach a position of about 78 above the horizon...not 90.  The zenith is the point directly overhead at 90.

{This position depends on your latitude.  To determine the maximum altitude of the Sun in the sky on the summer solstice, subtract your latitude from 90 and add 23.5 degrees, the tilt of the Earth's axis.}


At 23.5 N latitude, a position on the Earth called the Tropic of Cancer, the Sun appears directly overhead at noon on the summer solstice.  At the time of  the winter solstice, the Sun appears directly overhead at 23.5 S latitude, the Tropic of Capricorn.  Along the Equator, on or about March 21 and September 22, the sun reaches the zenith on equinox days, days of "equalness" in which there are twelve hours of day and twelve hours of night.  These days mark the beginning of  two of our seasons, spring and fall.

Well to get back to the ancient peoples...unlike ourselves, the ancients did not have calendars, nor day planners, nor Palm Pilots to keep themselves organized.  Neither were they inside as much as we....but like us, they needed to eat to live, and there were no grocery stores!  Therefore, the planning and planting of crops was a critical decision, with life and death consequences.  Our ancestors might have wondered, "Is it spring yet? If we plant crops now, can we be pretty sure of no more winter weather?  However, if we plant too late, the crops might not mature before the first frosts of autumn; the crops would be ruined, and we'll starve."  So all over the planet, ancient peoples observed the sun to predict the seasons in order to survive.

(Slide: Stonehenge)

This slide is of an ancient structure built in England called Stonehenge. Even today we are not entirely sure how ancient people raised these huge stones and set them into position.

(Slide: Heel stone)

On the day of the Summer Solstice, an observer standing in the middle of Stonehenge, sighting through two stone "posts", would have seen the Sun rising above the Heel Stone, which is beyond the posts.

(Slide: Woodhenge at Cahokia)

Not every ancient observatory is made of stone. At Cahokia, Illinois, the site of a Woodhenge, an observer stands again at the center of the "henge". A large post was placed at a particular point some distance from the observer so as to line up with the rising sun on the summer solstice. Other ancient astronomers marked the rising or setting Sun with particular points on the horizon, such as mountain peaks.

(Slide: El Castillo at Chichen Itza) When the first European explorers came to the New World they were surprised to find elaborate structures like El Castillo at Chichen Itza, Mexico. El Castillo, an American pyramid which rivals the Egyptian ones, has four sides. With ninety-one steps on a side, El Castillo is a calendar of sorts, 4 times 91 is 364, plus the one step on the top of the pyramid makes a total of 365, as in 365 days of the year. On the equinoxes, the rising sun creates a shadow serpent along the edge the steps.

Another structure in the area looks a lot like a modern-day observatory (Slide: El Caracol). Of course, there was no telescope here, but the windows are constructed so that on particular days the rising of the sun or Venus, an important celestial object for the Maya people, could be seen. There was also a small pool of water just outside the doors of the building which was used as a sundial and planetarium. For sundial usage during the day, the Maya simply placed a stick in the middle of the pool and watched the position of the shadow change throughout the day. In the evening, the sky was relected in the pool and observers could more comfortably watch "the sky".

(Slide: Aztec Calendar) This calendar heavily influenced by the Maya people, kept track of the earth's yearly cycle, and incorporated many astronomical observations as well.

(Slide: Sun Dagger Structure) Again, not every observatory was elaborate. This one, in the southwestern part of the United States, is constructed of three large stone slabs wedged upright with smaller stones. On the day of the summer solstice, a dagger of light cast by the rising sun bisects a spiral (slide: spiral with light) carved into the rock behind the stones. On the winter solstice, two daggers of light frame the spiral.

Many cultures throughout the ages, had people who were specialists, knowers of natural cycles. These people, the experts at reading the signs of the seasons and keeping track of them for everyone else, were called shamen or priests. We have such people in our society today, only now we call them scientists!

To go part II of the Planetarium Program, click here.

To go part III of the Planetarium Program, click here.

To return to the Slide Show Index, click here.

To return to the Sun in Time home page, click here.